The invention relates to a method and an arrangement for a vibration-resistant playback device having a reduced number of returns and having a reduced power consumption. The field of application of vibration-resistant playback devices applies, in particular, to the portable and mobile use of playback devices of optical information media, in which uninterrupted and undisturbed reproduction of information must be ensured despite vibrations, the field of application not being restricted, however, to optical information media.
Optical scanning devices, in particular, are sensitive to mechanical vibrations, since the optical scanning system can be pitched from the track by vibrations and, as a result, the read operation of the information stored on the information medium is interrupted. During stationary operation, these vibrations are so small that they can be adequately damped by means of mechanical precautions and electrical control loops. However, in the case of mobile use, such as, for example, in a car, or in the case of portable devices, the abovementioned measures no longer suffice, and use is made of a buffer which buffers the data stream for the purpose of reproduction during an interruption of the reading of the information medium. A buffer of this type is known in accordance with the data sheet of the CXD2511Q/R circuit from Sony (Vibration-Protective Memory Controller for CD Players). In order to provide a data reserve, the data are read from the CD at twice the speed and are stored in the buffer. At the same time, data are read at normal speed from the buffer for the purpose of reproduction. In order to prevent an overflow of the buffer, the reading of the information medium or the writing of data to the buffer must be continuously interrupted and the read operation must be resumed after the occupancy of the buffer has fallen below a minimum occupancy. For this purpose, the scanning or reading system must be returned to the location of the interruption and the reading of the information medium must be continuously resumed. This sequential pattern of events is continued unremittingly provided that no vibration occurs. As a result, a high number of resets or returns of the scanning or reading system are already necessary during vibration-free operation. If a vibration occurs during a reading pause, no special measures are necessary.
However, if the vibration occurs during the read operation, then the filling of the buffer must be stopped immediately and the scanning system must be returned as quickly as possible to the location at which the read operation was interrupted, in order to read anew data from the information medium or the CD. Following successful conclusion of this operation, the filling of the buffer is resumed. Consequently, this jumping or resetting of the scanning system to the location of the interruption of the reading of the CD is not only necessary when vibrations occur but also necessary when vibrations do not occur in the vibration resistance mode of operation, since the read operation which is carried out at twice the speed must be interrupted when the buffer is full.
In order to write data from the information medium to the buffer following an interruption in accordance with the order present on the information medium, the reading of the CD is resumed using data prior to the last subcode address evaluated as valid by a microprocessor, and in order to start the writing to the buffer, or for the sequentially correct resumption, a comparison between the stored and the read audio data is carried out. This audio data comparison, which controls the resumption of the writing to the buffer in the event of a positive result, is carried out after the decoding and error correction and directly before the digital/analog conversion. The buffer is connected to the de-interleaving and error correction unit of the decoder and is arranged directly before the digital/analog converter.
In addition to the relatively high number of returns when no vibrations occur, a problem of this solution is the reliability of the data synchronization for starting the write operation to the buffer. The synchronization which is based on the audio data comparison is not always successful, since, owing to errors, the data bits do not always appear in the same way and accurately timed. A chronological offset also occurs when comparing very soft passages, in which only zeros are compared, or when comparing passages which contain multiple repetitions such as, for example, a sinusoidal tone at a specific frequency. The data comparison which must be carried out frequently in connection with the interruptions does not guarantee the determination of the exact synchronization instant, leads to jumps in the audio data and consequently, owing to chronological jumps in the title or disk time, does not ensure accurately timed reproduction corresponding to the stored information.
A further problem is constituted by the power consumption associated with the doubled reading speed and the frequent track jumps. The service life of batteries in portable devices is reduced approximately by half due to the decoding at twice the speed and an increased number of actuator control operations. This increased power consumption occurs even during operation without any vibrations, since the reading of the CD at twice the speed requires an interruption of the write operation and resetting or reversing of the scanning unit. Moreover, a DRAM must be provided as the buffer for the most precise data comparison possible, and it represents a considerable cost factor.
The strength to withstand shocks and vibrations acting on the device from the outside is of crucial significance for the functionality of portable CD players. In order to enjoy CDs when jogging, it is known to provide a "digital shock absorber", cf. Walter Schild: A digital shock absorber in order to enjoy CDs when jogging, in the daily newspaper Die Welt, dated Jul. 16, 1992. In order to permit undisturbed and uninterrupted reproduction of information despite a brief interruption in the scanning of information from the CD as a result of a shock which causes the laser or scanning beam to lose the information track, the CD is driven at twice the speed and data read from the information medium are buffered. Data are then available from the buffer for the undisturbed reproduction of information during the period of an interruption. This property of portable CD players is therefore also referred to as so-called "shock-proof memory" or "shock-protection memory". The disk which is driven at twice the speed supplies data twice as fast as is necessary for the reproduction, with the result that the buffer is rapidly filled with data once more even after an interruption. In this case, however, the driving rotational speed is not constant, since CDs are read at a constant linear velocity, the so-called CLV. Since the CD has a smaller number of pits in the inner region of the disk than in the outer region, for one revolution, the driving rotational speed of the disk turntable is correspondingly reduced when reading from the inner to the outer region in order to ensure a constant reading speed. In this case, a constant reference frequency corresponding to the reading speed is used to control the rotational speed. The current speed information is taken from the frequency of the signal scanned from the information medium, which frequency forms the so-called actual value in the control loop.
If the buffer is completely full, the reading in of data is interrupted until a predetermined occupancy which warrants the further reading in of data is reached. During this time, too, the information medium is driven at twice the speed and the scanning speed is controlled by the control loop to a constant linear velocity CLV. The interruption of the read-in operation causes frequent track jumping and frequent rotational speed changes even without the occurrence of disturbances, since the scanning of the information medium following an interruption of the read operation must be resumed at a point at which the read operation was previously interrupted. The frequent track jumping and the driving of the information medium at twice the speed result in an increased power requirement, which has a particularly disadvantageous effect on the total playing time which can be attained by batteries. The total playing time which can be attained using a set of batteries in shock-proof operation is reduced approximately by half in comparison with normal operation at normal scanning speed on account of the drive at twice the speed, decoding of the scanning signal at twice the speed and an increased number of movements of the actuator or scanning system owing to increased track jumping.
Therefore, the object of the invention is to reduce the number of returns and to avoid the abovementioned disadvantages of the known solution and to provide a vibration-resistant playback device which has a low power consumption despite ensuring the "shock-proof memory" function.
This object is achieved by means of the features specified in independent claims. Advantageous refinements and developments are specified in dependent claims.
The invention is based on a vibration-resistant playback device which is operated predominantly at normal playback speed despite ensuring protection against vibrations, and in which there is no interruption of the read operation from the information medium in the event of operation without any vibrations, as a result of which frequent track jumping is avoided and the power consumption is considerably reduced. Furthermore, the reliability and accuracy of the data synchronization following a vibration-induced interruption can be increased, and use can be made of a buffer which is less expensive than a DRAM. These advantages are achieved by means of an arrangement and a method for controlling the reading speed from the information medium as a function of a predetermined occupancy of the buffer below its maximum storage capacity, the control arrangement varying the reading speed from the information medium in order to store a predetermined amount of data in a buffer without interrupting the reproduction before said data is read from the buffer. Overflowing of the buffer or the situation where the occupancy of the buffer falls below a minimum occupancy is prevented without interrupting the reading from the information medium by the fact that a reading speed which is increased in order to fill the buffer is already reduced to normal speed at a predetermined occupancy of the buffer below its maximum storage capacity, and the number of returns is reduced thereby. In contrast with reading the information medium at twice the speed, a reading speed in the region of the normal speed is set, which speed is preferably the conventionally used normal speed in playback devices without special vibration protection, when the occupancy of the buffer has not fallen below a predetermined occupancy. Only during the filling of the buffer is use made of a reading speed which is above the normal speed, but preferably without any loss in the bit timing of the phase control, until a predetermined occupancy of the buffer is reached. Resetting or returning of the scanning or reading unit is no longer necessary in order to produce the preconditions for vibration resistance, as a result of which the power consumption is considerably reduced. The increase in the reading speed without any loss in the bit timing which is required for processing the information from the information medium is made possible by a bit timing PLL having a relatively large capture range. This capture range is preferably more than .+-.60%. In order to reduce frequent changing over of the reading speed in connection with the occupancy of the buffer falling below and exceeding a predetermined occupancy, and in order to reduce the power consumption further, a first and a second occupancy are preferably used as the predetermined occupancy. If the buffer is full, it becomes necessary to change over the reading speed only in the event of vibration, since the read-in and read-out speeds are identical. The occupancy changes at normal speed. The reading speed is then accelerated when the occupancy falls below a first occupancy, and is not reduced until the second occupancy value is reached. In this case, the second occupancy value is selected in a manner analogous to control as a function of only one occupancy in such a way that the buffer is likewise filled only to its maximum capacity during the transition from an increased speed to the normal speed. The use of a first and a second occupancy value results in a hysteresis which leads to greater running smoothness. The control arrangement is preferably implemented as part of the decoder, and the buffer is preferably arranged between the EFM demodulator (EFM=eight to fourteen modulation) and the de-interleaving and error correction unit which provides the digital information. This arrangement enables simple processing of subcode information such as, for example, information regarding time and location, which then only have to be processed at normal speed in the decoder and can advantageously be used for the data synchronization. Decoding at twice the speed is unnecessary, since the buffer acts as a time base compensation element. The subcode can also be read and evaluated at a variable reading speed in the PLL capture range. The time information, that is to say the title or disk time corresponding to precisely that music which is to be heard, is calculated from the instantaneously read subcode and the occupancy. The already known data comparison with the disadvantages specified could also be used for the synchronization or for the writing of data to the buffer without any gaps. However, it is advantageous to use for the synchronization subcode data, preferably the synchronization word or synchronization bits, which are present in the data stream. Although the subcode data are available at different points in time, particularly as a result, too, of the reading speed which is adapted to the occupancy of the buffer, and only occur at relatively large spacings, reliable synchronization and a seamless data flow following an interruption are achieved by the fact that use is made of an item of address information which identifies the interruption location prior to the error correction in the data stream. The starting point of the synchronization is formed by a valid subcode, the address information of which can be used as a reference both for the location of the interruption on the information medium and for the last valid data in the buffer and the writing to the buffer. Using the address which identifies the location of an interruption on the information medium and in the buffer, the synchronization and the writing to the buffer after an interruption can also be implemented even before the address of the interruption, since the spacing between the last valid address, which corresponds to the synchronization byte of the last valid subcode, and the new address, which corresponds to the synchronization byte of the new valid subcode, can be calculated. The data following the address are then overwritten, resulting in a shorter time to synchronization. For this purpose, the buffer is preferably arranged upstream of the error correction and de-interleaving unit.
A data comparison is not carried out and a seamless data flow is, nevertheless, reliably achieved, despite a varying reading speed, particularly as a result of the fact that the buffer simultaneously serves as a time base compensation unit and data are read at a constant speed from the buffer.
On account of the arrangement of the buffer between the EFM demodulator and the de-interleaving and error correction unit which provides the digital audio information, it is possible to use an ARAM, which is less expensive than a DRAM. The use of an ARAM or audio RAM, which may be partially defective, is made possible by the arrangement before the error correction.
In addition to reducing the number of returns or the number of repositionings of the scanning or reading system, also referred to as a pick-up, in vibration-resistant playback devices by controlling the reading speed of the information medium as a function of the occupancy of the buffer, further aspects of the invention comprise the rapid availability of data for the reproduction after a start and the data synchronization. The number of necessary synchronization operations is already reduced by the smaller number of returns and their accuracy and reliability can be increased further. In order to reduce the power consumption of vibration-resistant playback devices, a power supply-reducing means is furthermore provided, by means of which the power supply to the drive motor of the information medium is temporarily interrupted or at least reduced. If a stationary information medium and special drive means influencing the reading speed were to be provided, then the power supply of said means could be correspondingly interrupted or reduced. Moreover, in contrast with the fundamental reading at twice the speed, the information medium is read at an increased speed only for the purpose of filling the buffer, and the known vibration resistance is, nevertheless, reliably achieved. Since filling of the buffer is necessary only after a restart or only after a vibration-induced interruption, the information medium can be read or driven predominantly at the normal reading speed, which corresponds to the normal speed. The operation at normal speed already results in a considerably reduced power consumption, since, on the one hand, the rotational speed of the motor is correspondingly lower and, in addition, control operations and movements of the scanning system for the purpose of finding the scanning location after the interruption due to the risk of the buffer overflowing as a result of writing at twice the speed and reading at normal speed are dispensed with. In order to reduce the rotational speed, not only is the reference frequency or the desired rotational speed value in the known CLV control loop reduced, which would still likewise require a high power consumption owing to a so-called hard rotational speed changeover, but rather it is provided to interrupt the power supply to the drive motor or at least significantly reduce it. The means provided for this purpose is preferably an electronic switch provided in the power supply line to the drive motor.
By means of the reference signal source, which can be varied in a predetermined range, for controlling the rotational speed of the drive motor, the scanning speed is set as a function of the occupancy of the buffer in such a way that writing or reading-in of data to the buffer is provided at an increased speed compared to the reading-out speed from the buffer only when the occupancy of the buffer falls below a predetermined value at the starting instant of the playback operation or on account of the track being lost. During the periods in which no track is lost and the buffer has a predetermined occupancy, the information medium is driven only at normal speed. As a result of this type of rotational speed control, scanned data are no longer read into the buffer at a constant speed or at a constant frequency. Nevertheless, the reproduction of information is not disturbed if a phase-locked loop or a so-called PLL having a large capture range is provided. Varying the reading speed without any loss in the bit timing required for the processing of the information from the information medium is made possible by means of a bit timing PLL which has a relatively large capture range. This capture range should preferably be more than .+-.60%. Therefore, the reproduction signal or the reproduction of information is not influenced by fluctuations in the reading speed or in the rotational speed of the drive motor of the information medium given a correspondingly large capture range in the range, for example, of from 80% to 180% of the normal value. This large capture range, which, inter alia, can also be attributed to the eight to fourteen modulation used for the data storage or data recovery, permits data to be read into the buffer at varying speeds, with the result that the rotational speed of the drive motor of the information medium can be varied within the capture range of the PLL in order to reduce the power consumption. Since the rotational speed of the drive motor can be returned to normal speed after a predetermined occupancy of the buffer has been reached, it is preferably provided, for the purpose of reducing power consumption, to disconnect the drive motor completely from the power supply during the period until the normal speed or a predetermined speed value, which may also be less than the normal speed, is reached. This disconnection is achieved by causing the connection to have a high resistance. The resulting so-called soft changeover of the rotational speed of the drive motor leads, in conjunction with the reduction in the doubled speed to the normal speed, to a considerably reduced power consumption, which permits considerably longer operation of the playback device with a set of batteries.
Although the above explanations were essentially referring to a CD as the information medium, the field of application is not restricted thereby, since in principle all digital latches and information media, including, for example, the digital videodisk or digital tapes, have a comparatively similar data structure, which enables the principle upon which the invention is based to be applied.